A bipolar battery construction includes an electrically conductive bipolar layer, so called biplate, that serves as electrical interconnection between adjacent cells in the battery as well as a partition between the cells. For efficient utilization of the bipolar construction, the biplate should be sufficiently conductive to transmit current from cell to cell, chemically stable in the cell's environment, capable of making and maintaining good contact to the electrodes and capable of being electrically insulated and sealable around the boundaries of the cell so as to contain electrolyte in the cell.
The above is more difficult to achieve in rechargeable batteries due to the charging potential that can generate gas inside the battery, and in alkaline batteries due to the creep nature of electrolyte. Achieving a desired combination of these characteristics has proven very difficult.
For maintenance-free operation, it is desirable to operate rechargeable batteries in a sealed configuration. However, sealed bipolar designs typically utilizes flat electrodes and stacked-cell constructions that are structurally poor for containment of gases present and generated during cell operation. In a sealed construction, gases generated during charging should be chemically recombined within the cell for stable operation. The pressure-containment requirement creates additional challenges in the design of a stable bipolar configuration.
New desires in the field of transportation, communications, medical and power tools are generating specifications that existing batteries cannot meet. These include higher cycle life and the need for rapid and efficient recharges.
NiMH systems are seen as the alternative to meet cycle life, but costs for existing conventional fabrication are too high.
In U.S. Pat. No. 5,344,723 by Bronoel et al., a bipolar battery is disclosed having a common gas chamber, which is created by providing an opening through the biplate (conductive support/separator). The opening is also provided with a hydrophobic barrier to prevent passage of electrolyte through the hole. Although a problem with pressure differences between the cells may be avoided, there is still a disadvantage with the described battery. The outer sealing around the edge of each biplate still has to be fluid-tight, which is very difficult to achieve. If the outer sealing is not fluid-tight, the electrolyte, contained in the separator between the electrodes and in the electrodes, may migrate from one cell to another.
In the published international patent application Ser. No. WO 03/026042 A1 assigned to the present applicant, the entire content of which is hereby incorporated herein by reference, a different solution compared to the solution described in U.S. Pat. No. 5,344,723, is proposed where a hydrophobic barrier is introduced around the electrodes instead of around the opening in the biplate. A pressure relief valve is also introduced to prevent a too high pressure to build up inside the case. It may, however, be rather expensive to manufacture a bipolar battery of this design.